1. Field of the Invention
This invention relates to silver halide photographic materials and, particularly, to silver halide photographic materials using chemically sensitized high-speed silver halide emulsions.
2. Description of the Prior Art
The production step of silver halide photographic emulsions generally includes "ripening", which is typically divided into a "pre-ripening" (i.e., a formation step of silver halide and a physical ripening step thereof) and a "post-ripening" (i.e., a chemical ripening step).
As a sensitizing process in the pre-ripening process, a process is known using a solvent for silver halides. Silver halide solvents generally used can include ammonia, potassium thiocyanate, and thioether compounds, but these compounds do not always have sufficiently satisfactory properties.
For example, the use of ammonia causes the defects that "fog" is liable to form and the material may give off an offensive smell. Thioether compounds are also liable to cause fog. Potassium thiocyanate causes problems from a toxicity point of view.
On the other hand, in sensitizing processes involving the post-ripening step, many chemical sensitization processes are known.
For instance, a sulfur sensitization process is known wherein a compound containing sulfur capable of reacting with silver ions or active gelatin is used, a reduction sensitization process is known wherein a reducing substance is used, and a noble metal sensitization process is known wherein a gold compound or other noble metal compounds are used. These processes can be used individually or in combination with each other.
Sulfur sensitizers that can be used include thiosulfates, thioureas, thiazoles, rhodanines, and other various compounds. Practical examples of such sensitizers are described in U.S. Pat. Nos. 1,574,944, 2,410,689, 2,278,947, 2,728,668, 3,656,955, 4,030,928, and 4,067,740.
As reduction sensitizers, stannous salts, amines, hydrazine derivatives, formamizinesulfinic acids, silane compounds, etc., can be used, and practical examples thereof are described in U.S. Pat. Nos. 2,487,850, 2,419,974, 2,518,698, 2,983,609, 2,983,610, 2,694,637, 3,930,867, and 4,054,458.
For noble metal sensitization, gold complex salts, as well as other complex salts of metals belonging to group VIII of the Periodic Table, such as platinum, iridium, and palladium, can be used, and practical examples thereof are described in U.S. Pat. Nos. 2,399,083 and 2,448,060, and British Pat. No. 618,061.
It has been strongly desired to increase the sensitivity of silver halide emulsions as high as possible, and for that purpose, attempts to increase the sensitivity of silver halide emulsions by adding further a new sensitizing technique to a well-known sensitizing technique have been made.
For example, U.S. Pat. Nos. 3,625,697, 3,622,329, and 3,574,709 disclose the technique of using a wellknown sensitization technique together with certain organic thioether compounds.
However, these attempts are not always satisfactory and suffer from the defects, for example, that fog is excessively increased, the extent of the progress of chemical ripening is changed by adding said new sensitizing technique (which makes it difficult to control the extent of chemical ripening), and when silver halide emulsions are stored in a refrigerator, etc., in a solidified state, by cooling during the period from chemical ripening to coating, the properties thereof are degraded. Also, the sensitivity itself is not always satisfactory.